1. Field of the Invention
The present invention relates to a system for optically and electronically ascertaining the existence and nature of precipitation and for measuring rain, snow, and drizzle in the ambient air from a remote location.
2. Description of the Prior Art
A light emitting diode weather identification system has previously been developed to remotely detect the existence of precipitation and to determine the nature of precipitation. That is, the prior weather identification system can differentiate between rain and snow. This prior weather identification system was established with the intent of automating surface weather observations at airports. The ability to completely automate surface observations is critically dependent on the ability to automatically distinguish between rain and snow. This prior system is disclosed in U.S. Pat. No. 4,760,272 issued on Jul. 26, 1988.
The prior weather identification system is able to correctly identify different types of precipitation in a near field region by transmitting a partially coherent light source over a path length much shorter than that of prior optical precipitation systems. According to the system of U.S. Pat. No. 4,760,272 a partially coherent light source, such as an infrared light emitting diode, may be used in place of a laser source which conventional optical weather identification systems previously required. The partially coherent beam of light is transmitted over a short distance of less then one meter and is detected by an optical receiver located in communication with the transmitter and in spaced separation therefrom.
The percentage light intensity fluctuations (scintillation indices) detected by the optical receiver within certain frequency ranges are indicative both of the existence of precipitation and the nature of the detected precipitation. Frequency components above one kilohertz are indicative of rain. Snow induced frequencies are primarily at a few hundred hertz. Accordingly, electronic signals generated from the received scintillations are directed to at least two different band pass filters to quantify scintillations in low and high frequency ranges. Scintillation signals in the low frequency range indicate snow while scintillations in the higher frequency range indicate rain. The low frequency band may be from twenty five hertz to two hundred fifty hertz, for example, while the high frequency band may extend from one kilohertz to four kilohertz.
Because high frequency components of rain drop induced scintillations are critical for discriminating between rain and snow, a half angle of incoherency which is too large may result in an insufficient number of usable signals in the high frequency band. Therefore, for all practical purposes the product of one half the angle of incoherency of the transmitter, as measured in radians, multiplied by the distance of the transmitter and receiver, must be no greater than about 2.5 millimeters.
While the system of U.S. Pat. No. 4,760,272 quite accurately determines the level of rain and snow, it does not provide an indication of drizzle. As a result of increasingly acute environmental problems, a need existed for accurate sensors to measure drizzle in the atmosphere.
A system was developed to provide a reliable visibility sensor that operates on the principle of forwardscatter in combination with the weather identification system of the type described in U.S. Pat. No. 4,760,272. The visibility sensor of this later development is described in U.S. Pat. No. 5,444,530. The device disclosed in that patent is able to utilize signals from the weather identification portion of the system to neutralize many of the ambiguities that arose in the measurement of light transmission that are unrelated to visibility.
However, it was later learned that even the system of U.S. Pat. No. 5,444,530 had certain shortcomings. Specifically, it is not sensitive enough to detect and identify light drizzle or to distinguish light drizzle from fog. However, according to the present intention, it has been discovered that, with certain modifications, an improved version of that system can be utilized to reliably detect and correctly identify even extremely light drizzle and to distinguish extremely light drizzle from fog.